This invention relates to the recovery method of ammonia from a gaseous mixture containing ammonia and carbon dioxide.
A gaseous mixture containing ammonia and carbon dioxide, is generated in a process in which dialkyl carbonate is produced from urea and an aliphatic alcohol, and a process in which urea is synthesized from ammonia and carbon dioxide.
For example, in the process where dialkyl carbonate is synthesized from urea and an aliphatic alcohol, water contained in urea as a raw material reacts with urea to generate ammonia and carbon dioxide, and to generate the gaseous mixture in which ammonia carbon dioxide and the vapor of an aliphatic alcohol are contained. And, in order to recover ammonia from such gaseous mixture, it is necessary to separate carbon dioxide and the accompanying aliphatic alcohol from the gaseous mixture.
In addition, for example, when synthesizing urea from ammonia and carbon dioxide, the gaseous mixture containing unreacted ammonia and unreacted carbon dioxide is exhausted. And to recover ammonia from this gaseous mixture, it is necessary to separate out carbon dioxide.
However, under a temperature less than 50xc2x0 C. at atmospheric pressure, the ammonia and carbon dioxide in the gaseous mixture react and generate a solid ammonium carbamate. For this reason, ammonium carbamate is deposited on the surface in an apparatus, and causes problems of reducing the efficiency of the processing, or causing a blockade of the piping.
Furthermore, when ammonia is recovered from the gaseous mixture containing ammonia, there is a problem that the amount of ammonia recovered from a gas component is reduced as a result of the formation of ammonium carbamate.
Moreover, since ammonium carbamate is a solid when it is generated, there is a problem that the recovery operation becomes complicated because it cannot be dealt with as a fluid.
In order to solve such problems, JP-B-61-8013 discloses separating ammonia as a gas when the solubility of ammonia became superfluous (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese publicationxe2x80x9d) by absorbing the gaseous mixture containing ammonia and carbon dioxide with a solvent, and heating the solution in a temperature range from which the solubility of carbon dioxide does not become superfluous.
However, in this method, it is necessary to circulate a large quantity of solvent in order to be able to make use of the change in saturation solubility of ammonia in the solvent by changing the temperature. In addition, a lot of energy is required for cooling and for heating of the solvent which absorbed ammonia, and to reuse it.
Furthermore, when a gaseous mixture contains organic compounds, such as an alcohol, the organic compounds can not be separated only by such a process, and another facility is needed, or there is a problem that separation itself is difficult.
In JP-A-6-115928 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), a method in which ammonia is recovered from an exhaust gas containing ammonia, carbon dioxide, and an organic compound is proposed. Under conditions where an ammonium carbamate does not generate, removing the carbon dioxide from an exhaust gas as sodium carbonate is proposed by washing the gas in sodium hydroxide aqueous solution.
However, in this method, a lot of chemicals and energy are needed for processing since alkali-salt aqueous solution, such as sodium carbonate, was produced so much. As a result, a large amount of facility cost and running cost are required.
Conventionally, as a method of recovering ammonia from a gas containing ammonia and carbon dioxide, the process as shown in the flow diagram of FIG. 2 is adopted industrially. That is, in the method shown in FIG. 2, the gaseous mixture containing ammonia and carbon dioxide is supplied to a carbon dioxide absorption tower 31 through a line 51. It is washed by a sodium hydroxide aqueous solution introduced from a line 57, and the carbon dioxide in a gaseous mixture is absorbed by sodium hydroxide aqueous solution to form a sodium carbonate.
A mixed aqueous solution of sodium hydroxide and sodium carbonate is extracted from the bottom of the tower of a carbon dioxide absorption tower 31 and introduced into a stirring tub 34 through a line 53 and a line 58.
Hydrochloric acid is supplied to a stirring tub 34, and the remaining sodium hydroxide is neutralized and discharged from a line 59 as a drain.
On the other hand, the gas in which ammonia concentration is increased is obtained from the top of the tower of a carbon dioxide absorption tower 31 through the line 52, which is concentrated in a multi-stage process to obtain liquefied ammonia from a line 66.
However, in such method, concentration of ammonia is required a multi-stage process, and resulted in a problem that a large amount of facility and energy were needed.
Moreover, since it is necessary to use an alkali solution in a large amount and to use an acid solution for neutralization in a large amount, there was a problem that many chemicals were needed for discharge of carbon dioxide.
For this reason, a method of separating ammonia efficiently from the gaseous mixture containing ammonia and carbon dioxide, without generating an ammonium carbamate was strongly desired.
The present invention is directed to a method of carrying out recovery of the ammonia efficiently from the gaseous mixture containing ammonia and carbon dioxide, without generating a solid ammonium carbamate.
The ammonia recovery method from a gaseous mixture of the present invention comprises a process (I) in which the gaseous mixture containing ammonia and carbon dioxide is contacted with an organic solvent to allow the organic solvent absorb the ammonia in the gaseous mixture, and a process (II) in which the organic solvent which absorbed ammonia is distilled to separate ammonia from the organic solvent.
In the present invention, in the above-mentioned process (I), it is desirable to reuse the organic solvent separated in the above-mentioned process (II).
In addition, it is also desirable that the organic solvent of the present invention is one or more organic solvents selected from the group consisting of an alcohol, ether, ketone, ester, and a halogenated hydrocarbon. It is more desirable that the organic solvent is an alcohol, and it is still more desirable that it is an aliphatic alcohol having carbon numbers of 3 to 6.
Furthermore, it is desirable that the gaseous mixture of the present invention is a mixture containing ammonia, carbon dioxide, and an alcohol, and that the gaseous mixture is an exhaust gas produced in a process in which urea and an alcohol are reacted.
In the case of the gaseous mixture of the present invention contains an alcohol, the organic solvent uses the present invention is preferably the same.